Boot a KVM guest on capable targets as a baseline test

On capable hosts when the kernel has KVM support it would be good to add a baseline tests that verifies that the kernel we just booted on metal can host itself as a KVM guest. We should be able to do this fairly straightforwardly with kvmtool, it has an option for using plan9 to share the host filesystem with the guest which makes life a lot easier.

This could of course be built on very easily, with both testing interoperation of different kernel versions and running suitable testsuites on the guests (the latter should be easier to add on if we're using the target filesystem